Time-of-flight mass spectrometers are known for their high transmission, good mass resolution, and fast analysis time. They are therefore potentially advantageous in situations that require fast mass spectral acquisitions, such as in fast gas chromatography/mass spectrometry (GC/MS) analyses, compared to conventional mass spectrometers, such as quadrapole mass filters and ion trap mass spectrometers.
In order to perform mass analysis of gas molecules, e.g., the effluent from a GC, the gas molecules must first be ionized, which is the function of an ion source. An efficient ion source will convert as many sample molecules into ions as possible and produce an optimal beam for the type of analyzer being used. The most common type of ion source for GC/NMS instruments is an `electron ionization source`. In this type of source, the gaseous sample stream is introduced into a chamber, which is itself contained in the evacuated housing of the mass spectrometer. Electrons are typically produced by thermal emission from a hot filament located outside the chamber. The electrons are accelerated through an electric field to a particular and relatively homogeneous energy, as defined by the potential difference between the filament and the ion source chamber. This is typically 70 eV, but can vary from about 10 eV to upwards of 150 eV. The electrons are directed into and through the chamber. When an electron collides with a sample gas molecule in the chamber, one possible (desirable) result is that the gas molecule loses an electron and therefore becomes a positively charged ion. Once the sample molecule acquires a charge, it can respond to electrostatic fields that accelerate it out of the ion source and guide it into the entrance of the mass spectrometer.
In the case of a time-of-flight mass spectrometer, the entrance region consists of a pulsed acceleration region, in which an electrostatic field can be turned on and off with fast transitions. While this electrostatic field is kept off, ions from the ion source are allowed to enter this acceleration region. When the electrostatic field is turned on, the effect of the field causes the ions to be accelerated into a field-free flight tube of the mass spectrometer, where they travel until they reach a detector or mass analyzer. Sometimes an electrostatic mirror is deployed after some distance along the flight tube, in which the ions reverse direction, and continue through a second segment of field-free flight tube before reaching the detector or mass analyzer. Because the ions are accelerated to the same nominal energy, their flight velocity will be proportional to the square root of their mass. Over the fixed (effective) length of the flight tube, then, the measured spectrum of charge intensity vs. arrival time represents the mass spectrum of ions initially contained in the acceleration region. This mass spectrum is obviously related to the relative concentrations of ions in the ion source, which, in turn, reflects the chemical constituents in the gaseous effluent from the gas chromatograph (or other source of gas to be mass analyzed).
It is most advantageous that the ions enter the time-of-flight acceleration region traveling in a direction that is orthogonal to the time-of-flight flight tube axis. Ions in the acceleration region will be accelerated in a direction parallel to the flight tube axis and perpendicular to the ions' initial direction of travel. Since the time-of-flight acceleration region is of a limited dimension along the ions' initial direction of travel, only ions within the boundaries of this region will enter the flight tube and be analyzed. After this analysis cycle has completed, the field in the acceleration region is turned off, and the beam of ions from the source is then allowed to enter the acceleration region. Then, again, at a pre-determined time, the field is pulsed on and the analysis cycle repeats. The spectrum from each individual cycle could be preserved separately, but, typically, several hundreds of such mass spectra are acquired and integrated to increase the signal/noise characteristics.
Now, GC/MS applications frequently require that ion intensities be measured over a signal dynamic range of up to six or seven orders of magnitude. This results from the fact that signal intensities from the different ion masses present at any one time can typically extend over several orders of magnitude from one mass ion to another, and, in addition, ion intensities will vary over time as the chromatographic effluent gas concentration varies in the ion source by several orders of magnitude. While the detectors and acquisition electronics of conventional quadrapole mass filters are capable of realizing such dynamic range performance, the specialized detectors and acquisition electronics necessary for time-of-flight mass spectrometry are currently not able to achieve this amount of dynamic range with any one fixed setting of the gain in the detection system. That is, when the overall gain in the time-of-flight detection system is adjusted so that the smallest signal levels of interest (i.e., a single ion of any mass) are measurable, then the highest signals, which also need to be accommodated, will saturate the detection system, and hence will not be measurable under these gain conditions. Similarly, if the gain in the detection/acquisition system is adjusted so that the largest signals of interest are accommodated, then signals of interest in the lower intensity ranges will not be detectable.
Obviously, one approach to accommodate all signal levels of interest with time-of-flight mass spectrometers is to adjust the gain of the time-of-flight detector between spectral acquisitions by adjusting its voltage. In this way, a composite spectrum could be constructed by combining the individual spectra acquired with different gain settings. There are at least two difficulties with this approach: 1) the gain vs. detector voltage relationship would have to be well known and stable in order for the measurement to be quantitative, and this would be difficult on a routine basis because of the non-linear, and variable, relationship between the gain of a detector and the applied detector voltage; and, 2) in order to be compatible with `fast` spectral acquisitions, the voltage changes would have to occur at the .about.2 kV level with relatively sharp transition and settling times, which would involve significant additional complexity and expense.
Another approach to accommodate a wider range of signal levels would be to vary the ion source electron beam current. That is, when intense signals are present, the electron beam current could be reduced, and the probability that a gas molecule is ionized is correspondingly reduced. Similarly, when the mass peaks of interest are weak, the electron beam current could be increased to effectively increase the ionization probability, or efficiency.
There are at least two difficulties with this approach: 1) for the measurements to be interpreted with an acceptable degree of quantification requires accurate and precise control over the electron beam current. Such control would be achieved by measuring the electron beam current, and using this measurement in a `feedback` loop, to regulate the emission from the electron source filament, either by adjusting the filament current, or by adjusting the voltage on a control grid electrode near the filament, in a well known fashion. The problem here is that the response time of such feedback schemes is much slower, typically of the order of tenths of a second or longer, depending on the electron current being measured, than would be required to be compatible with `fast` chromatographic time resolutions, which would commonly be of the order of tens of milliseconds or less. 2) Another problem arises from the fact that the electron beam, which consists of negative charges, distorts electrostatic fields along and around its path. In the ion source chamber, gas molecules are ionized by collisions with the electron beam and the ions are directed out of the chamber by a weak electrostatic field. This initial extraction field is weak causing a small energy divergence in the ion beam, and in turn, the electron beam introduces a small but significant distortion of this weak electrostatic field. The resulting ion beam is subsequently controlled by electrostatic focusing optics. Optimization of these optics depends sensitively on the energy and angular emission characteristics of the ion beam as it leaves the source chamber, which, in turn, depends on the detailed spatial dependence of the electrostatic field in the chamber. Provided that the electron beam current is constant, the distortion of the field will be constant, and the down-stream focusing optics can be adjusted to take the effect of this distortion on ion trajectories into account. However, if the electron beam current is adjusted as described above to accommodate a wider range of signal intensities, the result would be a variable distortion of the electrostatic field in the ion source, which would degrade the quality of the focusing of the ion beam.
An additional problem sometimes occurs in GC/MS and other similar instruments that the most intense mass peaks in the mass spectrum originate from chemical species in the sample gas that are of no interest in the analysis, such as from the GC carrier gas, solvent species, or other unimportant constituents. Often, such intense mass peaks can interfere with the quality of the analysis, for example, due to possible detector saturation and recovery problems, amplifier overload, space charge effects in the mass analyzer, etc. Such intense mass peaks are eliminated in the current art by introducing an electrostatic gate in the flight tube of the time-of-flight mass analyzer. Such gates are activated to prevent unwanted ions from reaching the detector. They usually involve an array of fine wires in the flight path, and, as such, have the disadvantages of: 1) reducing the transmission of the analyzer; 2) introducing surfaces in the flight path which eventually become contaminated with a thin insulating layer, and so may exhibit charging and degrade performance; and, 3) additional mechanical and electronic complexity and expense.